Development of Blue Phosphorescent Pt(II) Materials Using Dibenzofuranyl Imidazole Ligands and Their Application in Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) are energy-efficient; however, the coordinating ligand can affect their stability. Sky-blue phosphorescent Pt(II) compounds with a (CN)-N-boolean AND chelate, fluorinated-dbi (dbi = [1-(2,4-diisopropyldibenzo [b,d]furan-3-yl)-2-phenyl-1H-imidazole]), and acetylactonate (acac) (1)/picolinate (pic) (2) ancillary ligands were synthesized. The molecular structures were characterized using various spectroscopic methods. The Pt(II) Compound Two exhibited a distorted square planar geometry, with several intra-and inter-molecular interactions involving C-pi center dot center dot center dot H/C-pi center dot center dot center dot C-pi stacking. Complex One emitted bright sky-blue light (lambda(max) = 485 nm) with a moderate photoluminescent quantum efficiency (PLQY) of 0.37 and short decay time (6.1 mu s) compared to those of 2. Theoretical calculations suggested that the electronic transition of 1 arose from ligand((CN)-N-boolean AND)-centered pi-pi* transitions combined with metal-to-ligand charge-transfer (MLCT), whereas that of 2 arose from MLCT and ligand((CN)-N-boolean AND)-to-ligand(pic) charge-transfer (LLCT), with minimal contribution from (CN)-N-boolean AND chelate to the lowest unoccupied molecular orbital (LUMO). Multi layered phosphorescent OLEDs using One as a dopant and a mixed host, mCBP/CNmCBPCN, were successfully fabricated. At a 10% doping concentration of 1, a current efficiency of 13.6 cdA(-1) and external quantum efficiency of 8.4% at 100 cdm(-2) were achieved. These results show that the ancillary ligand in phosphorescent Pt(II) complexes must be considered.